1. Technical Field
The present invention relates to a method for manufacturing a rotor of a rotating electric machine that is, for example, mounted in a vehicle, and used as a motor or a generator.
2. Related Art
A motor with a squirrel-cage rotor is known in related art as a type of rotating electric machine used to be mounted in a vehicle or the like. The squirrel-cage rotor has a squirrel-cage structure with conductors having both axial ends that are short-circuited together. The squirrel-cage rotor includes a rotor core and a conductive member.
The rotor core is composed of a plurality of steel plates that are stacked in an axial direction of the rotor. The plurality of steel plates have a center shaft hole and a plurality of through holes. The center shaft hole passes through the steel plates in the axial direction. The plurality of through holes pass through the steel plates in the axial direction and are arrayed in a circumferential direction of the rotor.
The conductive member has a pair of end rings and a plurality of connection bars. The pair of end rings are disposed on both axial ends of the rotor core in the axial direction. The plurality of connection bars connect the pair of end rings through the through holes. The conductive member is integrally formed by casting.
A method for manufacturing a squirrel-cage rotor in related art such as that described above involves a setting step and a casting step. At the setting step, a plurality of steel plates configuring a rotor are stacked in an axial direction of the rotor and set in a predetermined position in a mold. At the casting step, molten metal is fed into a molten metal introduction passage, thereby forming a conductive member. The molten metal introduction passage has a gate that opens onto one axial end side of the stacked steel plates that are set in the mold.
In this method, as shown in FIG. 24, the molten metal is introduced from a gate 124a of a molten metal introduction passage 124 into an end ring cavity 123a on one axial end side of the set stacked steel plates. The introduced molten metal then flows into the plurality of through holes 113 provided in the stacked steel plates 111a, in the order from a through hole 113a, which is located at a position nearest to the gate 124a in a radial direction D2, to a through hole 113b which is located at a position furthest from the gate 124a in the radial direction D2. Therefore, the molten metal flowing into the through hole 113a reaches an end ring cavity 123b on the other axial end side of the set stacked steel plates first.
The molten metal flowing from the through hole 113a then reaches, via the other axial end side, the through hole 113b ahead of the molten metal that flows into the through hole 113b from the one axial end side. As a result, the flow of molten metal from the other axial end side merges with the flow of molten metal from the one axial end side. A problem occurs in that a cold shut may be thereby formed.
In addition, as shown in section A in FIG. 25, a problem also occurs in that a blowhole may be formed as a result of air within the mold becoming trapped in a connection bar 117 that is formed within the through hole 113b. When the blowhole and the above-described cold shut are formed in this way, properties, such as strength and conductivity, of the conductive member are significantly affected.
Therefore, JP-A-563-73852 proposes improving the balance of flow of the molten metal that flows through the through holes in the rotor core. The improvement is made by a cylindrical ring being provided at the axial end portion of the pair of end rings disposed on both axial end sides of the rotor core. The cylindrical ring has a radial-direction thickness that is thinner than the end ring.
In addition, JP-A-S60-204244 proposes a technique for improving the balance of flow of the molten metal that flows through the through holes in the rotor core. The technique involves providing a plurality of gates in the circumferential direction. The gates each open into the end ring cavity on the one axial end side of the stacked steel plates that are set in the mold.
However, in the case of above-described JP-A-S63-073852, a casting defect caused by solidification shrinkage of the molten metal easily occurs in areas in which the thickness of the end ring is increased. In addition, when a cutoff process is performed to ensure product shape after completion of the casting step, a problem occurs in that the casting defect is exposed on the surface.
On the other hand, in the case of above-described JP-A-S60-204244, the plurality of gates that open into the end ring cavity are evenly disposed in the circumferential direction. However, there is a limit to the number of gates that can be disposed. Although the balance of flow is improved compared to when the molten metal flows in from the end portion of the end ring as in the past, described above, the flow is not completely even.
Furthermore, in the case of JP-A-S60-204244, when the gates are cut off after completion of the casting step, tensile stress between the gate portion and the product part is used to cut off the gates. Therefore, a large load is also applied to the product part. The gate portion is required to be made smaller to prevent the large load from being applied to the product part. However, when the gates are made smaller, the fluidity of the molten metal becomes extremely poor. A problem occurs in that casting defects easily occur because casting pressure becomes difficult to apply.